Problem Drake

Hi folks,
I have a slight problem with one of our drakes. If I let him out in the garden with the ducklings he goes straight for them. I don't know if he wants to teach them who is boss, kill them or try and mate with them. But I don't want to find out the hard way. The other drake is fine with them and is younger. I can let the ducklings, the ducks and all the chickens run around the garden with no problems. So I am wondering if I should pen him up on his own and put the rest together as the chickens and ducks live together, or will it make him worse in the long run?.
We have two drakes four ducks and six ducklings, all the adults are khaki campbell cross with some unknown breed. Four of the ducklings are ment to be Appleyards but I think there is a bit of khaki campbell in there somewhere and the other two are ment to be Khaki campbell cross Indian runner, all of which are 8 weeks old or there abouts they were sold to us as 12 day old ducklings.

I don't want to lose any and although they all have there feathers they don't have their quack yet. So should I wait for them to quack or put Daddy duck on his own.